1. Field of the Invention
The present invention relates to a distance measuring semiconductor integrated circuit as a main component of a distance measuring unit mounted on an active or passive type automatic focusing device.
2. Description of the Related Art
The distance measuring unit mounted on an automatic focusing device, when the unit is of an active type, normally includes a light-emitting diode for emitting light to a subject, where the distance to the subject is to be measured, a PSD (Position Sensitive Detector) element for receiving the light reflected by the subject and outputting a current according to the angle of incidence of the light, and a distance measuring integrated circuit (distance measuring IC) for converting the current supplied by the PSD element from an analog form to a digital form and then computing data (distance measurement data) corresponding to the distance to the subject.
In such distance measuring units as described above, fine adjustments of the optical system are performed physically in order to correct measuring errors due to dispersion in the fabrication of distance measuring ICs and the displacement of the optical system (a light-detecting element, a light-emitting element, etc.).
However, the measuring errors are very difficult to correct completely by the above-mentioned adjustment of the optical system. Therefore, an electrical method is used to eliminate the measuring errors. More specifically, the distance measuring IC is provided with two to three terminals for external connections with variable resistances dedicated to adjustments and by adjusting the values of those adjusting resistances at the initial stage of use, the current is modified at the position of the A-D converter to thereby eliminate errors of the distance measurement data. This correction method using the adjusting resistances requires the resistance values to be adjusted by repeating the distance measurement a number of times until output data becomes the desired distance measurement data. As a result, a large amount of labor is required.
As a method of eliminating errors of measurement data without using adjusting resistances, a correcting method is known in which an independent memory is attached externally to an ordinary distance measuring IC and this memory is used to store a linear function obtained by measuring known distances with this distance measuring IC. The problems with this method are that not only is an externally-attached memory used but also a circuit for generating timing pulses for adjustment is required. Also, wiring needs must be added to interconnect the elements, and that, above all, a burden of preparing software for the CPU becomes very large.